Lateral MOSFET with dielectric isolation trench

ABSTRACT

A lateral trench MOSFET comprises an insulating layer buried in a substrate, a body region in the substrate, an isolation region in the substrate, a first drain/source region over the body region, a second drain/source region in the substrate, wherein the first drain/source region and the second drain/source region are on opposing sides of the isolation region, a drift region comprising a first drift region of a first doping density formed between the second drain/source region and the insulating layer, wherein the first drift region comprises an upper portion surrounded by isolation regions and a lower portion and a second drift region of a second doping density formed between the isolation region and the insulating layer, wherein a height of the second drift region is equal to a height of the lower portion of the first drift region.

This application is a continuation of U.S. patent application Ser. No.13/415,965, entitled “Lateral MOSFET with Dielectric Isolation Trench,”filed on Mar. 9, 2012, which application is incorporated herein byreference.

BACKGROUND

The semiconductor industry has experienced rapid growth due toimprovements in the integration density of a variety of electroniccomponents (e.g., transistors, diodes, resistors, capacitors, etc.). Forthe most part, this improvement in integration density has come fromshrinking the semiconductor process node (e.g., shrink the process nodetowards the sub-20 nm node). As semiconductor devices are scaled down,new techniques are needed to maintain the electronic components'performance from one generation to the next. For example, lowgate-to-drain capacitance, low on-resistance and high breakdown voltageof transistors are desirable for high power applications.

As semiconductor technologies evolve, metal oxide semiconductor fieldeffect transistors (MOSFET) have been widely used in today's integratedcircuits. MOSFETs are voltage controlled devices. When a control voltageis applied to the gate of a MOSFET and the control voltage is greaterthan the threshold of the MOSFET, a conductive channel is establishedbetween the drain and the source of the MOSFET. As a result, a currentflows between the drain and the source of the MOSFET. On the other hand,when the control voltage is less than the threshold of the MOSFET, theMOSFET is turned off accordingly.

MOSFETs may include two major categories. One is n-channel MOSFETs; theother is p-channel MOSFETs. According to the structure difference,MOSFETs can be further divided into three sub-categories, planarMOSFETs, lateral double diffused MOS (LDMOS) FETs and vertical doublediffused MOSFETs. In comparison with other MOSFETs, the LDMOS is capableof delivering more current per unit area because its asymmetricstructure provides a short channel between the drain and the source ofthe LDMOS.

In order to further improve the performance of the LDMOS, an isolationtrench may be added into a lateral MOSFET to increase the breakdownvoltage of the lateral MOSFET. In particular, the gate region, thechannel region and the drift region of the lateral MOSFET are formedalong the sidewall of the isolation trench. Such a lateral trench MOSFETstructure helps to reduce the on-resistance as well as increase thebreakdown voltage of lateral MOSFETs.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a simplified cross-sectional view of a lateral trenchMOSFET in accordance with an embodiment;

FIG. 2 illustrates a cross section view of a semiconductor device aftera dielectric layer is applied to the substrate in accordance with anembodiment;

FIG. 3 illustrates a cross section view of the semiconductor deviceshown in FIG. 2 after an etching process is applied to the semiconductordevice in accordance with an embodiment;

FIG. 4 illustrates a cross section view of the semiconductor deviceshown in FIG. 3 after a thin dielectric layer is formed in the trench302 and the trench 304 in accordance with an embodiment;

FIG. 5A illustrates a cross section view of the semiconductor deviceshown in FIG. 4 after an anisotropic etching process is applied to thetrench 302 and the trench 304 in accordance with an embodiment;

FIG. 5B illustrates a cross section view of the semiconductor deviceshown in FIG. 5A after an extra anisotropic etching process is appliedto the trench 302 and the trench 304 in accordance with an embodiment;

FIG. 6 illustrates a cross section view of the semiconductor deviceshown in FIG. 5B after bottom dielectric layers are formed at thebottoms of the trench 304 and the trench 304 respectively in accordancewith an embodiment;

FIG. 7 illustrates a cross section view of the semiconductor deviceshown in FIG. 6 after an isotropic etching process is applied to thetrench 302 and the trench 304 respectively in accordance with anembodiment;

FIG. 8 illustrates a cross section view of the semiconductor deviceshown in FIG. 7 after dielectric materials are filled into the trenchesshown in FIG. 7 in accordance with an embodiment;

FIG. 9 illustrates a cross section view of the semiconductor deviceshown in FIG. 8 after an anisotropic etching process is applied to thesemiconductor device shown in FIG. 8 in accordance with an embodiment;

FIG. 10 illustrates a cross section view of the semiconductor deviceshown in FIG. 9 after a thin liner oxide layer is formed on thesidewalls of the trench shown in FIG. 9 in accordance with anembodiment;

FIG. 11 illustrates a cross section view of the semiconductor deviceshown in FIG. 10 after a gate electrode material is filled in thetrenches in accordance with an embodiment;

FIG. 12 illustrates a cross section view of the semiconductor deviceshown in FIG. 11 after a chemical mechanical polish (CMP) process or anetch-back process is applied to the top surface shown in FIG. 11 inaccordance with an embodiment;

FIG. 13 illustrates a cross section view of the semiconductor deviceshown in FIG. 12 after an anisotropic etching process is applied to thetop surface of the semiconductor device in accordance with anembodiment;

FIG. 14 illustrates a cross section view of the semiconductor deviceshown in FIG. 13 after body regions are formed in the substrate inaccordance with an embodiment; and

FIG. 15 illustrates a cross section view of the semiconductor deviceshown in FIG. 14 after drain/source regions are formed over thesubstrate in accordance with an embodiment.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the embodimentsof the disclosure, and do not limit the scope of the disclosure.

The present disclosure will be described with respect to embodiments ina specific context, a lateral metal oxide semiconductor field effecttransistor (MOSFET) with a dielectric isolation trench. The embodimentsof the disclosure may also be applied, however, to a variety of metaloxide semiconductor transistors.

FIG. 1 illustrates a simplified cross-sectional view of a lateral trenchMOSFET in accordance with an embodiment. The lateral trench MOSFET 100includes a substrate with a first conductivity and an insulating layer101 buried in the substrate. More particularly, the substrate can bedivided into two portions. As shown in FIG. 1, an upper substrateportion 102 is formed over the insulating layer 101; a lower substrateportion 103 is formed below the insulating layer 101. In accordance withan embodiment, the insulating layer 101 is formed of silicon dioxide.The substrate may be a lightly doped n-type substrate, which is formedby implanting n-type dopants such as phosphorous at a concentration ofbetween about 5×10¹⁶/cm³ and about 9×10¹⁶/cm³. The substrate shown inFIG. 1 is commonly referred to as a silicon-on-insulator substrate.

A first drain/source region 112 and a second drain/source region 114 areformed in the upper substrate portion 102 over the insulating layer 101.Isolation regions 104 and 106 are formed between two active regions. Forexample, as shown in FIG. 1, the isolation region 104 is formed betweenthe first drain/source region 112 and the second drain/source region114. In accordance with an embodiment, the first drain/source region 112is a drain of the lateral trench MOSFET 100 and the second drain/sourceregion 114 is a source of the lateral trench MOSFET 100.

The first drain/source region 112 is formed in the upper substrateportion 102. In accordance with an embodiment, the first drain/sourceregion 112 functions as a drain of the lateral trench MOSFET 100. Thefirst drain/source region 112 may be formed of n-type dopants. The drainregion may be formed by implanting an n-type dopant such as phosphorousat a concentration of between about 1×10¹⁹/cm³ and about 5×10¹⁹/cm³.

The second drain/source region 114 is formed in a body region 122. Inaccordance with an embodiment, the second drain/source region 114 may bea source of the lateral trench MOSFET 100. The source region may beformed by implanting an n-type dopant such as phosphorous at aconcentration of between about 1×10¹⁹/cm³ and about 5×10¹⁹/cm³. As shownin FIG. 1, the source region is formed adjacent to the isolation region104 on the opposite side from the drain (the first drain/source region112).

The lateral trench MOSFET 100 further comprises the body region 122 witha second conductivity formed in the upper substrate portion 102 over theinsulating layer 101. As shown in FIG. 1, the body region 122 is formedunderneath the second drain/source region 114. In accordance with anembodiment, when the substrate is n-type, the body region 122 is ap-type body region. The body region 122 is formed by implanting p-typedoping materials such as boron, gallium, aluminum, indium, combinationsthereof, or the like. In accordance with an embodiment, a p-typematerial such as boron may be implanted to a doping density of about10¹⁷/cm³ to 3×10¹⁸/cm³. Alternatively, the body region 122 can be formedby a diffusion process. The body region 122 of the lateral trench MOSFET100 may be alternatively referred to as a channel region.

The lateral trench MOSFET 100 may comprise a gate 142. As shown in FIG.1, the gate 142 is enclosed by a dielectric layer. In particular, thedielectric layer separates the gate 142 from the second drain/sourceregion 114. In accordance with an embodiment, the gate 142 may becoupled to a control signal. When the control signal is greater than thethreshold voltage of the lateral trench MOSFET 100, the lateral trenchMOSFET 100 is turned on. On the other hand, when the control signal isless than the threshold voltage, the lateral trench MOSFET 100 is turnedoff accordingly.

The lateral trench MOSFET 100 may comprise a drift region comprising afirst drift region 116 formed between the first drain/source region 112and the insulating layer 101 and a second drift region 118 formedbetween the isolation region 104 and the insulating layer 101. Inaccordance with an embodiment, the first drift region 116 is an n-typeregion having a doping concentration in a range from about 10¹⁷/cm³ toabout 5×10¹⁷/cm³. The second drift region 118 is an n-type region havinga doping concentration in a range from about 10¹⁶/cm³ to about3×10¹⁷/cm³.

The dimensions of the depth of the isolation region 104 and the gapbetween isolation region 104 and the insulating layer 101 are shown inFIG. 1. In particular, the depth of the isolation region 104 is definedas H1. The gap between the isolation region 104 and the insulating layer101 is defined as H2. In accordance with an embodiment, H1 isapproximately equal to 1 um. H2 is in a range from about 0.05 um toabout 0.3 um.

One skilled in the art will recognize that FIG. 1 illustrates an idealprofile. The dimensions of H1 and H2 may vary after subsequentfabrication processes. H1 and H2 shown in FIG. 1 are used to illustratethe inventive aspects of the various embodiments. The disclosure is notlimited to any particular dimensions of H1 and H2.

The isolation regions (e.g., isolation region 104) are used to improvethe breakdown voltage of the lateral trench MOSFET 100. In particular,as shown in FIG. 1, the bottom surface of the isolation region 104 isadjacent to the insulating layer 101. Both the insulating layer 101 andthe isolation region 104 are formed of dielectric materials such assilicon dioxide. The proximity between two silicon dioxide layers maylead to a fully depleted second drift region 118. Such a fully depleteddrift region helps to reduce the electric field at the surface of thelateral trench MOSFET 100 during off-state. Likewise, the first driftregion 116 may be fully depleted because it is located between twosilicon dioxide regions 104 and 106. As such, the fully depleted seconddrift region 116 helps to reduce the electric filed at the surface ofthe lateral trench MOSFET 100.

The influence of the fully depleted drift region (e.g., second driftregion 118) is similar to the effect of reduced surface field (RESURF).RESURF is a well-known mechanism to improve the breakdown voltage ofhigh voltage MOSFETs. As such, the fully depleted drift regions can helpto improve the breakdown voltage of the lateral trench MOSFET 100.Moreover, because the breakdown voltage of the lateral trench MOSFET 100is improved, a highly doped drift region may be employed to furtherreduce the on-resistance of the lateral trench MOSFET 100. In sum, thefully depleted drift region 118 helps to improve the breakdown voltageas well as the on-resistance of the lateral trench MOSFET 100.

One advantageous feature of a lateral trench MOSFET with a dielectricisolation trench (e.g., isolation region 104) is that the trenchstructure shown in FIG. 1 helps to improve the breakdown voltage as wellas the on-resistance of the lateral trench MOSFET 100. In other words,the trench structure helps to maintain the breakdown voltage of alateral trench MOSFET. In addition, the trench structure can reduce theon-resistance of the lateral trench MOSFET 100 so that the power lossesof the lateral trench MOSFET 100 may be reduced. Furthermore, thelateral trench structure of FIG. 1 may help to reduce the pitch of thelateral trench MOSFET 100. Such a reduced pitch may help to reduce thechannel length as well as the turn-on resistance of the lateral trenchMOSFET 100.

FIGS. 2-15 illustrates cross section views of intermediate steps offabricating a lateral trench MOSFET in accordance with an embodiment.FIG. 2 illustrates a cross section view of a semiconductor device aftera dielectric layer is applied to the substrate in accordance with anembodiment. As shown in FIG. 2, a dielectric layer 132 is formed on topof an upper substrate portion 102 over an insulating layer 101. Asdescribed above with reference to FIG. 1, the substrate may be an n-typeSOI substrate.

The dielectric layer 132 may be formed of various dielectric materialscommonly used in integrated circuit fabrication. For example, thedielectric layer 132 may be formed of silicon dioxide, silicon nitrideor a doped glass layer such as boron silicate glass and the like.Alternatively, dielectric layer may be a layer of silicon nitride, asilicon oxynitride layer, a polyamide layer, a low dielectric constantinsulator or the like. In addition, a combination of the foregoingdielectric materials may also be used to form the dielectric layer 132.In accordance with an embodiment, the dielectric layer 132 may be formedusing suitable techniques such as sputtering, oxidation and/or chemicalvapor deposition (CVD).

FIG. 3 illustrates a cross section view of the semiconductor deviceshown in FIG. 2 after an etching process is applied to the semiconductordevice in accordance with an embodiment. In accordance with anembodiment, a patterned mask (not shown), such as a photoresist maskand/or a hard mask, is formed on the dielectric layer 132 usingdeposition and photolithography techniques. Thereafter, an etchingprocess, such as a reactive ion etch (RIE) or other dry etch, ananisotropic wet etch, or any other suitable anisotropic etch orpatterning process, is performed to form trenches 302 and 304.

FIG. 4 illustrates a cross section view of the semiconductor deviceshown in FIG. 3 after a thin dielectric layer is formed in the trench302 and the trench 304 in accordance with an embodiment. The thindielectric layers 402 and 404 may be may be an oxide layer thermallygrown in the trench 302 and the trench 304 respectively. Alternatively,the thin dielectric layers 402 and 404 can be formed by other suitabletechniques such as sputtering, oxidation and/or CVD.

FIG. 5A illustrates a cross section view of the semiconductor deviceshown in FIG. 4 after an anisotropic etching process is applied to thetrench 302 and the trench 304 in accordance with an embodiment. Ananisotropic etching process is applied to the trench 302 and the trench304. By controlling the strength and direction of the etching process,the bottom of the thin dielectric layers 402 and 404 have been removedas a result.

FIG. 5B illustrates a cross section view of the semiconductor deviceshown in FIG. 5A after an extra anisotropic etching process is appliedto the trench 302 and the trench 304 in accordance with an embodiment.An extra anisotropic etching process is applied to the trench 302 andthe trench 304. By controlling the strength and direction of the extraetching process, as shown in FIG. 5B, the bottom portions of thedielectric sidewalls of the trench 302 and the trench 304 have beenremoved as a result.

FIG. 6 illustrates a cross section view of the semiconductor deviceshown in FIG. 5B after bottom dielectric layers are formed at thebottoms of the trench 304 and the trench 304 respectively in accordancewith an embodiment. The bottom dielectric layers 602 and 604 may be anoxide layer thermally grown in the trenches 302 and 304 respectively. Itshould be noted that the bottom dielectric layers 602 and 604 can beformed by other suitable techniques such as CVD.

FIG. 7 illustrates a cross section view of the semiconductor deviceshown in FIG. 6 after an isotropic etching process is applied to thetrench 302 and the trench 304 respectively in accordance with anembodiment. An anisotropic etching process is applied to the trench 302and the trench 304. The thin liner dielectric layers on the sidewalls ofthe trench 302 and the trench 304 have been removed as a result.

FIG. 8 illustrates a cross section view of the semiconductor deviceshown in FIG. 7 after dielectric materials are filled into the trenchesshown in FIG. 7 in accordance with an embodiment. In accordance with anembodiment, the isolation regions 802 and 804 may be formed by firstforming trenches and then filling the trenches with a dielectricmaterial. In order to polish the surface of the semiconductor deviceshown in FIG. 8, a planarization process, such as CMP or etch back step,may be performed to planarize an upper surface of the isolation regions802 and 804.

The trenches (shown in FIG. 7) are filled with a dielectric materialthereby forming the isolation regions 802 and 804 as illustrated in FIG.8. The dielectric material may comprise, for example, a thermaloxidation, a CVD silicon oxide or the like. It may also comprise acombination of materials, such as silicon nitride, silicon oxy-nitride,high-k dielectrics, low-k dielectrics, CVD poly-silicon or otherdielectrics.

FIG. 9 illustrates a cross section view of the semiconductor deviceshown in FIG. 8 after an anisotropic etching process is applied to thesemiconductor device shown in FIG. 8 in accordance with an embodiment. Apatterned mask (not shown), such as a photoresist mask and/or a hardmask, is formed on the top surface of the semiconductor device usingdeposition and photolithography techniques. An anisotropic etchingprocess is performed to form trenches 902 and 904.

FIG. 10 illustrates a cross section view of the semiconductor deviceshown in FIG. 9 after a thin liner oxide layer is formed on thesidewalls of the trench shown in FIG. 9 in accordance with anembodiment. The thin oxide layer may be thermally grown in the trenches902 and 904. The dielectric layer on the top surface prevents anyadditional oxidation on the top surface of the semiconductor device.

FIG. 11 illustrates a cross section view of the semiconductor deviceshown in FIG. 10 after a gate electrode material is filled in thetrenches in accordance with an embodiment. The gate electrode layer 1102may be formed of polysilicon. Alternatively, the gate electrode layer1102 may be formed of other commonly used conductive materials such as ametal (e.g., tantalum, titanium, molybdenum, tungsten, platinum,aluminum, hafnium, ruthenium), a metal silicide (e.g., titaniumsilicide, cobalt silicide, nickel silicide, tantalum silicide), a metalnitride (e.g., titanium nitride, tantalum nitride), dopedpoly-crystalline silicon, other conductive materials, combinationsthereof, or the like.

FIG. 12 illustrates a cross section view of the semiconductor deviceshown in FIG. 11 after a chemical mechanical polish (CMP) process or anetch-back process is applied to the top surface shown in FIG. 11 inaccordance with an embodiment. A planarization process, such as CMP oretch back step, may be performed to planarize an upper surface of thegate electrode layer 1102. As shown in FIG. 12, a portion of the gateelectrode layer 1102 has been removed as a result. As shown in FIG. 12,there may be two gates after the CMP process, namely a first gate 1202and a second gate 1204.

FIG. 13 illustrates a cross section view of the semiconductor deviceshown in FIG. 12 after an anisotropic etching process is applied to thetop surface of the semiconductor device in accordance with anembodiment. An anisotropic etching process is applied to the top surfacein accordance with an embodiment. As a result, the dielectric layer 132(not shown but illustrated in FIG. 2) has been removed.

FIG. 14 illustrates a cross section view of the semiconductor deviceshown in FIG. 13 after body regions are formed in the substrate inaccordance with an embodiment. Body regions 122 and 124 may be formed inthe upper substrate portion 102. In accordance with an embodiment, whenthe upper substrate portion 102 is a lightly doped n-type substrate, thebody regions 122 and 124 may be formed by implanting appropriate p-typedopants such as boron, gallium, indium or the like. Alternatively, in anembodiment in which the substrate 103 is an n-type substrate, the bodyregions 122 and 124 may be formed by implanting appropriate n-typedopants such as phosphorous, arsenic, or the like. In accordance with anembodiment, the doping density of the body regions 122 and 124 is in arange from about 10¹⁷/cm³ to about 3×10¹⁸/cm³.

FIG. 15 illustrates a cross section view of the semiconductor deviceshown in FIG. 14 after drain/source regions are formed over thesubstrate in accordance with an embodiment. The drain/source regions 112and 114 may be formed on opposing sides of the isolation regions (e.g.,isolation region 802). In accordance with an embodiment, thedrain/source regions (e.g., drain/source region 112) may be formed byimplanting appropriate n-type dopants such as phosphorous, arsenic, orthe like. In accordance with an embodiment, the doping density of thedrain/source regions (e.g., drain/source region 112) is in a range fromabout 10¹⁹/cm³ to about 5×10¹⁹/cm³.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

What is claimed is:
 1. A structure comprising: an insulating layerburied in a substrate; a body region in the substrate; an isolationregion in the substrate; a first drain/source region over the bodyregion; a second drain/source region in the substrate, wherein the firstdrain/source region and the second drain/source region are on opposingsides of the isolation region; a drift region comprising: a first driftregion of a first doping density formed between the second drain/sourceregion and the insulating layer, wherein the first drift regioncomprises an upper portion surrounded by isolation regions and a lowerportion, wherein a width of the lower portion of the first drift regionis greater than a width of the upper portion of the first drift region;and a second drift region of a second doping density formed between theisolation region and the insulating layer, wherein a height of thesecond drift region is equal to a height of the lower portion of thefirst drift region; a first dielectric layer formed over the substrate;and a gate formed over the first dielectric layer.
 2. The structure ofclaim 1, wherein: the first drain/source region is formed between thegate and the isolation region and the first drain/source region is indirect contact with a sidewall of the isolation region.
 3. The structureof claim 1, wherein: the first drain/source region is a source region;and the second drain/source region is a drain region.
 4. The structureof claim 1, wherein: the isolation region comprises an upper portionbetween the first drain/source region and the second drain/sourceregion, and a lower portion having a sidewall in direct contact with theupper portion of the first drift region.
 5. The structure of claim 4,wherein: a width of the lower portion of the isolation region is greaterthan a width of the second drift region.
 6. The structure of claim 1,wherein: the first doping density of the first drift region is greaterthan the second doping density of the second drift region.
 7. Thestructure of claim 1, wherein: the substrate is an N-type substrate; thebody region is a P-type region; and the first drain/source region andthe second drain/source region are N-type regions.
 8. The structure ofclaim 1, wherein: the first dielectric layer is between the gate and thefirst drain/source region.
 9. A device comprising: a first isolationregion and a second isolation region in a substrate; a drain regionbetween the first isolation region and the second isolation region; afirst source region in a first body region over the substrate, whereinthe first source region and the drain region are on opposing sides ofthe first isolation region; a second source region in a second bodyregion over the substrate, wherein the second source region and thedrain region are on opposing sides of the second isolation region; afirst drift region of a first doping density, wherein an upper portionof the first drift region is between the first isolation region and thesecond isolation region; a second drift region of a second dopingdensity formed underneath the first isolation region; a third driftregion of a third doping density formed underneath the second isolationregion; a first gate adjacent to the first source region, wherein thefirst source region is between the first gate and the first isolationregion and the first source region is in direct contact with a sidewallof the first isolation region; and a second gate adjacent to the secondsource region.
 10. The device of claim 9, further comprising: aninsulating layer buried in the substrate.
 11. The device of claim 10,further comprising: the second drift region is between the firstisolation region and the insulating layer; and the third drift region isbetween the second isolation region and the insulating layer.
 12. Thedevice of claim 9, further comprising: a first dielectric layer betweenthe first gate and the first source region; and a second dielectriclayer between the second gate and the second source region.
 13. Thedevice of claim 9, wherein: the second source region is between thesecond gate and the second isolation region.
 14. The device of claim 9,wherein: a lower portion of the first drift region is between the seconddrift region and the third drift region.
 15. A method comprising:embedding an insulating layer in a substrate; forming an isolationregion over the insulating layer; forming a body region over theinsulating layer in the substrate; implanting ions to form a first driftregion and a second drift region, wherein: the first drift region is ofa first doping density and between a drain region and the insulatinglayer; and the second drift region is of a second doping density andbetween the isolation region and the insulating layer; implanting ionsto form the drain region and a source region, wherein the source regionand the drain region are on opposing sides of the isolation region; andforming a gate structure adjacent to the source region, wherein thesource region is formed between the gate structure and the isolationregion, and wherein the source region is in direct contact with asidewall of the isolation region.
 16. The method of claim 15, furthercomprising: forming a dielectric layer between the gate structure andthe source region.
 17. The method of claim 15, wherein: the first driftregion has a higher doping density than the second drift region.
 18. Themethod of claim 15, wherein: a bottom surface of the body region ishigher than a bottom surface of the gate structure; the bottom surfaceof the body region is higher than a bottom surface of the isolationregion; and the bottom surface of the gate structure is higher than thebottom surface of the isolation region.